Realizing High Thermoelectric Performance in GeTe through Optimizing Ge Vacancies and Manipulating Ge Precipitates

Abstract

Recent vast reinvestigations on GeTe, a promising thermoelectric material at medium temperature, have triggered enormous enthusiasm in the thermoelectric community again. Intrinsically, GeTe, as a p-type semiconductor, possesses plenty of Ge vacancies, resulting in extremely high carrier concentration. Meanwhile, because of the vast off-stoichiometric Ge, both concomitant Ge precipitates and Ge vacancies will influence the electrical and thermal transport properties caused by phonons and carriers scattering. Herein, we report one effective way to optimize the thermoelectric performance of GeTe through manipulating Ge vacancies and Ge precipitates via successive continuous steps. First, the optimization of carrier concentration through Bi counter-doping led to an optimum carrier concentration of ∼2.79 × 10 20 cm −3 . Second, the Ge phase aggregation was successfully suppressed by heat treatment, which enhanced the electrical properties due to improved carrier mobility. Additionally, the Ge precipitates were uniformly distributed by heat treatment, which provided all-scale phonons scattering and effectively reduced the lattice thermal conductivity. Finally, combined with the significantly increased power factor (PF) and the decreased thermal conductivity, the maximum ZT of ∼1.9 at 723 K was eventually obtained in Ge 0.94 Bi 0.06 Te just through optimizing Ge vacancies and manipulating Ge precipitates by means of heat treatment.

Manipulating the Ge Vacancies and Ge Precipitates through Cr Doping for Realizing the High‐Performance GeTe Thermoelectric Material

“…Through water quenching and annealing, Ge precipitates in several submicron scale were found in nearly homogenous distribution in the sample, which displays much lower lattice thermal conductivity compared to the sample without heat treatment. [ 48 ] By manipulating Ge precipitates and Ge vacancies as well as decreasing hole carrier concentrations through Cr doping effect, a higher ZT value is obtained in the Cr doped GeTe.…”

“…A very recent study shows that heat treatment by quenching and annealing process could produce more uniformly distributed Ge precipitates, which even benefits the thermoelectric performance by provided all‐scale phonons scattering. [ 48 ] In addition, another research also demonstrated that those Ge precipitates have no apparent negative impact on the electronic transport properties. [ 49 ] Actually, the study of the effect of vacancies and precipitates on the transport properties of materials such as Zintls has been established to some extent.…”

High Power Factor and Enhanced Thermoelectric Performance in Sc and Bi Codoped GeTe: Insights into the Hidden Role of Rhombohedral Distortion Degree

“…[ 38–40 ] Due to the similarly beneficial band structure and strong phonon anharmonicity with PbTe arising from the same chemical bonding mechanism, [ 41,42 ] comparable high peak ZTs ≈2 have continuously been reported in the system of GeTe. [ 29,43–56 ] One important difference between these two compounds is that GeTe undergoes the structural phase transition from the low‐temperature rhombohedral to the high‐temperature cubic phase at a critical temperature around 700 K. [ 57,58 ] The rhombohedral structure refers to a unit cell with parameters ( a = b = c and α = β = γ < 90°), belonging to the hexagonal crystal family. It can be regarded as a slightly distorted rock‐salt lattice along the [111] direction with a interaxial angle less than 90°.…”

“…[ 53 ] As is known, suppressing the Ge vacancies is essential to ensure high charge carrier mobility that will be beneficial to achieve a high power factor. [ 47,50,66,72 ]…”

“…As above‐mentioned, this is due to the significantly decreased carrier concentration after Sc doping. Nevertheless, the charge carrier mobility still shows the increased tendency (Figure 4b), which may be related to the microstructural modulation, e.g., Ge precipitates, [ 50,53 ] Ge‐vacancies thin layer, [ 54 ] and nanoscale domains, [ 50,52,72 ] to reduce the grain boundary scattering for charge carriers. Compared to other dopants, the unusual high values of the charge carrier mobility of Sc‐doped samples are caused by the weak scattering from fewer Ge vacancies.…”